The present invention relates to an improved electrophotographic photoconductor.
Recently organic photoconductors are widely used for electrophotographic copying machines of the advantages of low cost, high productivity and non-environment pollution problems over other photoconductors.
Representative examples of organic electrophotographic photoconductors are photoconductive resins such as polyvinylcarbazole (PVK), charge transporting complex type photoconductors, such as PVK-TNF (2,4,6-trinitrofluorenone), pigment-dispersed type photoconductors such as phthalocyanine-binder composition, and function-separation type photoconductors in which a charge generating material and a charge transporting material are used in combination. Among these organic photoconductors, particular attention is paid to the function-separation type photoconductors particularly.
When such a function-separation type photoconductor is applied to the Carlson process, its chargeability is generally not high and its charge retention capability is so poor that the dark decay is large. Furthermore, the chargeability and charge retention capability are considerably degraded while in use, and accordingly the obtained image density becomes non-uniform and decreases while in use of the photoconductor. In particular, in the case of reverse development, toner deposition is apt to take place on the background of the developed images.
Furthermore, such a function-separation type photoconductor has the shortcoming that the chargeability is decreased by the light fatigue prior to exposure. This is probably because the light fatigue prior to exposure is mainly caused by the light absorbed by a charge generating material contained in the photoconductor, so that the longer the period in which the electric charges generated by the light absorption remain in the photoconductor in a movable state, and the greater the number of the generated electric charges, the greater the reduction in chargeability caused by the light fatigue prior to exposure. In other words, even if the photoconductor is charged when the electric charges generated by the light absorption still remain in the photoconductor, the electric charge at the surface of the photoconductor is neutralized by the residual carriers in the photoconductor as they move, so that the surface potential of the photoconductor is not increased until the remaining electric charges are consumed. Therefore, the elevation of the surface potential is delayed by such a period of time that corresponds to the light fatigue prior to exposure.
In order to eliminate the above-mentioned shortcomings, for example, the following undercoat layers have been proposed for use in the conventional electrophotographic photoconductors: intermediate layers comprising nitrocellulose based resins as disclosed in Japanese Laid-Open patent application Nos. 47-6341, 48-3544 and 48-12034; intermediate layers comprising nylon based resins as disclosed in Japanese Laid-Open patent application Nos. 48-47344, 52-25638, 58-30757, 58-63945, 58-95351, 58-98739, and 60-66258; intermediate layers comprising maleic acid based resins as disclosed in Japanese Laid-Open patent application Nos. 49-69332 and 52-10138; and an intermediate layer comprising a polyvinyl alcohol resin as disclosed in Japanese Laid-Open patent application No. 58-105155.
Furthermore, intermediate layers containing various electroconductive additives have been proposed for controlling the electric resistivity thereof, for example, an intermediate layer comprising carbon or chalcogen materials dispersed in a cured resin as disclosed in Japanese Laid-Open patent application No. 51-65942; an intermediate layer comprising a polymer thermally polymerized by use of an isocyanate-type curing agent with addition of a quaternary ammonium salt as disclosed in Japanese Laid-Open patent application No. 52-82238; a resinous intermediate layer with addition of an electric resistivity adjusting agent as disclosed in Japanese Laid-Open patent application No. 55-1180451; a resinous intermediate layer in which aluminum oxide or tin oxide is dispersed as disclosed in Japanese Laid-Open patent application No. 58-58556; a resinous intermediate layer with addition of organic metal compounds as disclosed in Japanese Laid-Open patent application No. 58-93062; resinous intermediate layers with addition of electroconductive particles as disclosed in Japanese Laid-Open patent applications Nos. 58-93063, 60-97363 and 60-111255; a resinous intermediate layer in which magnetite particles are dispersed as disclosed in Japanese Laid-Open patent application No. 59-17557; and resinous intermediate layers in which finely-divided particles of TiO.sub.2 and SnO.sub.2 are dispersed as disclosed in Japanese Laid-Open patent application Nos. 59-84257, 59-93453 and 60-32054.
The above-mentioned conventional electrophotographic photoconductors, however, still have the shortcomings that the chargeability is decreased while in repeated use, the rising rate of the surface potential at the initial charging is insufficient and the residual potential changes too much for use in practice.